Auxetic structures can enable an article to exhibit an expansion in a lateral direction, upon subjecting the article to a longitudinal stress or strain. Conversely, auxetic structures also exhibit a contraction in the lateral direction upon subjecting such an article to longitudinal compression. Such materials are understood to exhibit a negative Poisson's ratio. Synthetic auxetic materials have been known since 1987 and are, for instance, described in the U.S. Pat. No. 4,668,557, the entirety of which is incorporated herein by reference. The '557 materials were prepared as open-celled polymeric foam and a negative Poisson's ratio was obtained as a consequence of compressive deformation of the foam. More recently, auxetic materials have been provided in the form of polymer gels, carbon filled composite laminates, metallic foams, honeycombs and microporous polymers. Recent research suggests that auxetic behavior generally results from a cooperative effect between the material's internal structure (geometry) and the deformation mechanism it undergoes when submitted to stress. (Grima, J. N; Alderson, A; Evans, K. E., Auxetic behaviour from rotating rigid units, Physica Status Solidi B:242(3), 561-576, 2005. Yang, Wei; Li, Zhong-Ming; Shi, Wei; Xie, Bang-Hu; Yang, Ming-Bo, Review on auxetic materials, Jour. Mater. Sci, 39(10), 3269-3279, 2004.) This counter-intuitive behavior imparts many beneficial effects on the material's macroscopic properties that make auxetics superior to conventional materials in many applications.
Auxetic behavior is also scale-independent. Thus, a considerable amount of research has focused on the ‘re-entrant honeycomb structure’ which exhibits auxetic behavior when deformed through hinging at the joints or flexure of the ribs. Traditional textile technologies have been adopted for manufacturing fabric reinforcements for advanced polymer composites. Knitting in particular is well suited to the rapid manufacture of components with complex shapes due to their low resistance to deformation. The use of net-shape/near net-shape preforms is highly advantageous in terms of minimum material waste and reduced production time.
However, despite exceptional formability, knit structures are often characterized as having in-plane mechanical performance less than optimal, as compared to more conventional woven or braided fabric structures. This problem is associated with the limited utilization of fiber stiffness and strength of the severely bent fibers in the knit structure and the damage inflicted on the fibers during the knitting process. However, knitted performs for composites, built up of multiple layers of fabric, can exhibit better tensile and compressive strength, strain-to-failure, fracture toughness and impact penetration resistance, compared to laminates with only a single layer of fabric. (Leong, K. H., Ramakrishna, S., Huang, Z. M., Bibo, G. A., The potential of knitting for engineering composites, Composites: Part A, 31, 197, 2000.) Such benefits have been attributed to either increased fiber content, mechanical interlocking between neighboring fabric layers through nesting, or both.
As mentioned above, the negative Poisson's ratio effect is due to the geometric layout of the unit cell microstructure, leading to a global stiffening effect in many mechanical properties, such as in-plane indentation resistance, transverse shear modulus and bending stiffness. (Smith, C. W., Grima, J. N., and Evans, K. E., A novel mechanism for generating auxetic behaviour in reticulated foam: Missing rib foam model, Acta Materiala, 48, 4349-4356, 2000.) The highly looped fiber architecture of a knit fabric provides one approach to an auxetic fabric, in that the structure undergoes a significant amount of deformation when subjected to external forces. (Ugbolue, S. C. O., Relation between yarn and fabric properties in plain-knitted structures, Jour. Text. Inst., 74, 272, 1983.) In addition, the three-dimensional (3D) nature of knit fabrics provides some fiber bridging that facilitates opening mode fracture toughness, so improvements of up to an order of magnitude over those of glass prepreg and woven thermosets composites have been reported. Moderate improvements to the strength and stiffness of knit composites can be achieved by the incorporation of float stitches into basic architecture; weft-insert weft-knit fabrics and weft-insert warp-knit fabrics have been produced on flat-bed and warp knitting machines. 3D knit sandwich composites and 3D warp knit non-crimp composites are recent developments, but limited published information is available on their mechanical properties. Various researchers report that these composites have a higher energy absorption capacity, but exhibit lower flexural stiffness and specific compressive strength compared with several conventional sandwich polymer composites containing polymer (PMI) foam or Nomex™ cores. Overall, there remains in the art a need for an auxetic textile structure and method of fabrication, to better utilize the corresponding benefits and advantages.